This famous proposition from biology suggests that, if we have similar embryologic development, we are closely related.
The idea goes as follows. Embryogenesis is a developmental sequence at the earliest parts of life for a multicellular organism, an early way of structuring a mass of dividing cells into a body plan.
There is a sequence of operations in early development that specify important changes that later stages all build upon. Those later stages depend upon prior stages to have been established before they act. Natural selection might intervene here or there in that sequence, but it can make a far more reliable and survivable impact later in a developmental sequence, because it doesn't interfere with all of the previous development. Earlier stages in development for related species of organisms must thus be more conserved, an idea that tends to bear out within members of close taxonomic groups.
Taxonomies, when they are well-grounded, group organisms in hierarchies based on shared characteristics. Add notions of 'family trees', and you get 'phylogeny'. The basic urge of phylogeny is to understand common derivation. Convergent evolution of characteristics can complicate this analysis, but new genetic and developmental analyses have resolved a number of these confusions.
From this point of view, we know for sure that humans evolved from primordial fish and other earlier vertebrates. Human embryos first develop gill slits and a tail, both of which are usually resorbed as unnecessary when later human developmental programs kick in, all in utero. For a time, our embryos are also anatomically difficult to discriminate from a chicken embryo (and class Aves in general), or from a turtle embryo (order Chelonia), the latter a vertebrate derived very early on from other reptiles, apparently prior to the dinosaurs.
You are more like a turtle than you might realize.
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